Add Claude Code marketplace plugin support

- Add .claude-plugin/marketplace.json and plugin.json manifests
- Use ${CLAUDE_PLUGIN_ROOT:-$HOME/.claude/skills/last30days} for script path
- Plugin install: uses CLAUDE_PLUGIN_ROOT when set
- Manual install: falls back to ~/.claude/skills/last30days (no breakage)
- SKILL.md stays at root, plugin.json points skills to "./"

Users can now install via:
  claude plugin marketplace add mvanhorn/last30days-skill
  claude plugin install last30days@last30days

Inspired by PR #1 (galligan) — lighter approach without repo restructure.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -0,0 +1,16 @@
+{
+  "name": "last30days",
+  "owner": {
+    "name": "mvanhorn",
+    "url": "https://github.com/mvanhorn"
+  },
+  "metadata": {
+    "description": "Research any topic from the last 30 days across Reddit, X, and the web"
+  },
+  "plugins": [
+    {
+      "name": "last30days",
+      "source": "."
+    }
+  ]
+}

.claude-plugin/plugin.json

@@ -0,0 +1,12 @@
+{
+  "name": "last30days",
+  "description": "Research any topic from the last 30 days across Reddit, X, and the web",
+  "version": "2.0.0",
+  "author": {
+    "name": "mvanhorn"
+  },
+  "repository": "https://github.com/mvanhorn/last30days-skill",
+  "license": "MIT",
+  "keywords": ["research", "reddit", "twitter", "x", "trends", "prompts"],
+  "skills": ["./"]
+}